1. Field of the Invention
The present invention relates to a force sensor and a method for producing the force sensor, and particularly to a force sensor in which a positioning of a glass member is facilitated upon joining a force sensor chip and an attenuator through the glass member, and joint strength is prevented from being reduced when the force sensor chip and the attenuator are joined by anodic bonding, and a method for producing the force sensor.
2. Description of the Related Art
Conventionally, in an industrial robot and the like, there has been adopted a multi-axis force sensor for accurately measuring a size and direction of an external force applied to the robot during an action of the robot, in order to implement a control under which the robot appropriately and flexibly responds to the external force.
As the multi-axis force sensor, for example, there has been known a force sensor utilizing a property of a strain resistive element (piezo resistive element), in which a resistance value changes in accordance with minute strain (compression, tension) caused by an applied external force (See, for example, Japanese unexamined patent publication Nos. 2003-207405 and 2003-254843, the disclosures of which are herein incorporated by reference in their entireties).
The multi-axis force sensor has a force sensor chip formed on a semiconductor substrate by a semiconductor production process, and an attenuator made of a metal member for accommodating and securing the force sensor chip.
Specifically, in the force sensor chip, the strain resistive elements are appropriately arranged around an action portion to which an applied external force is transmitted, and a change in a resistance value of the strain resistive element due to the external force is detected as an electrical signal, which presents a size and direction of the external force. If the applied external force is directly transmitted to the strain resistive element, and the external force is excessively large, the force sensor chip may be damaged. In order to receive the external force of various magnitudes without causing damage, the attenuator is introduced for attenuating the applied external force to an appropriate magnitude to transmit to the force sensor chip.
Though various types of attenuators are present, in a typical attenuator, the force sensor chip is held from below by a fixing portion, and is joined to a lower fixing portion and an upper transmission portion in such a manner that an external force is transmittable from the upper transmission portion to the force sensor chip.
In this case, if the force sensor chip formed on the semiconductor substrate is directly joined with the attenuator made of a metal member, there arise problems, such as electric hazard including leakage from an electric source, detachment of two members at the joint portion due to a difference in coefficient of thermal expansion between two members, and thermal strain, all of which may deteriorate the detection accuracy.
In order to overcome the problems, some conventional techniques introduce a bulky (massive) glass plate, which has approximately the same thickness as that of the semiconductor substrate, as an interface between the force sensor chip and the attenuator, from a viewpoint of insulation property and coefficient of thermal expansion. For joining the glass plate to the force sensor chip and to the attenuator, an epoxy resin adhesive is applied to the joint faces thereof, or the joint faces are chemically bonded by anodic bonding. In anodic bonding, while the subject is heated, a voltage is applied to the subject with a negative voltage on a glass plate side and a positive voltage on an object to be joined, in order to transfer alkali ion, such as Na+, from the glass to the object. Typical thickness of the glass plate to be joined to the object is approximately 0.1 to several mm.
The conventional anodic bonding method will be specifically described with reference to FIGS. 19A-D. FIGS. 19A-D illustrates steps of anodic bonding at joint portions of a force sensor chip and an attenuator with the presence of a glass plate therebetween, in which FIG. 19A shows joining of the glass plate and the attenuator, FIG. 19B shows joining of the attenuator with the glass plate joined thereto and the force sensor chip, FIGS. 19C and 19D are cross sectional views showing a case where anodic bonding is applied to a specific attenuator, in which FIG. 19C shows joining of the glass plate and the attenuator, and FIG. 19D shows joining of the attenuator with the glass plate joined thereto and the force sensor chip.
In the anodic bonding, as described above, a voltage is applied to the subject with a negative voltage on the glass plate, and a positive voltage on an object to be joined. Accordingly, when the attenuator and the glass plate are joined at first, as shown in FIG. 19A, a voltage is applied to the subject with a negative voltage on the glass plate 100 and a positive voltage on the attenuator 300. Then, when a complex of the attenuator 300 with the glass plate 100 joined thereto and the force sensor chip 200 are joined by anodic bonding, as shown in FIG. 19B, a voltage is applied with a negative voltage on an attenuator 300 side and thus on a glass plate 100 side of the joint portion 600, and a positive voltage on the force sensor chip 200.
A specific example for the anodic bonding will be described with reference to a force sensor 1000. As shown in FIGS. 19C and 19D, for example, when the attenuator 300 is joined with a first glass member 110 and a second glass member 120 through a joint portion 510 and a joint portion 520, respectively, by anodic bonding (see FIG. 19C), and the force sensor chip 200 is joined with the first glass member 110 and the second glass member 120 through a joint portion 610 and a joint portion 620, respectively, by anodic bonding, anodic bonding is performed by applying a voltage with a negative voltage on a second glass member 120 side and a positive voltage on a force sensor chip 200 side (see FIG. 19D).
It should be noted that, in this explanation of the conventional anodic bonding, the attenuator 300 and the glass plate 100 are joined first, and then the glass plate 100 and the force sensor chip 200 are joined; however, there may be a case in which the force sensor chip 200 and the glass plate 100 are joined first and then the glass plate 100 and the attenuator 300 are joined.
However, when an epoxy resin adhesive is used at a joint portion of the attenuator and the force sensor chip, adhesion may become poor due to aged deterioration of the adhesive. In addition, a joint surface may be deformed or adhesive strength may become smaller, due to repeated compression and tension caused by external force on the attenuator. As a result, there arises a problem that minute change in external force cannot be accurately transmitted.
When the attenuator and the force sensor chip are joined by anodic bonding, and two portions including a fixing portion and a transmission portion are separately joined to the attenuator, a positioning step and joining step of the glass member should be performed for each of the fixing portion and the transmission portion, leading to a redundant number of steps. Especially, an action portion facing a center portion of the force sensor chip should be accurately positioned, though the subject to be positioned (the joint portion and the glass plate) are very small and thus the positioning frequently becomes difficult.
Further in anodic bonding, directions of voltage applied to the joint portion 500 are opposite between a case of joining of the attenuator 300 with the glass plate 100 (FIG. 19A), and a case of joining of the glass plate 100 with the force sensor chip 200 (FIG. 19B). As a result, fracture in the joint interface may occur from alkali ion (e.g., Na+) in the glass plate 100, leading to problems, such as reduced joint strength and detachment of the joint face.
These problems also occur in a case where first the force sensor chip 200 is joined with the glass plate 100 and then the glass plate 100 is joined with the attenuator 300.
The above-mentioned problems of the anodic bonding will be described with reference to a specific example of the force sensor 1000 as shown in FIGS. 19C and 19D.
In the force sensor 1000, as shown in FIG. 19C, first a voltage is applied with a positive voltage on an attenuator 300 side, and a negative voltage on a first glass member 110/second glass member 120 side, to thereby join the attenuator 300 and the first glass member 110 at the joint portion 510, and join the attenuator 300 and the second glass member 120 at the joint portion 520 by anodic bonding. In the case of this anodic bonding, an electron flow from the first glass member 110 and the second glass member 120 to the attenuator 300 is generated. It should be noted that the joint portions 510, 520 correspond to the joint portion 500 in FIG. 19A.
Subsequently, as shown in FIG. 19D, a voltage is applied with a positive voltage on the second glass member 120 side, and a positive voltage on the force sensor chip 200 side, to thereby join the first glass member 110 and the force sensor chip 200 at the joint portion 610, and join the second glass member 120 and the force sensor chip 200 at the joint portion 620, by anodic bonding. In the case of this anodic bonding, an electron flow is generated at the joint portion 620 from the second glass member 120 to the force sensor chip 200 (forward voltage), and at the same time, an electron flow e is also generated at the joint portion 610 from the second glass member 120 through the attenuator 300 to the joint portion 510. Since a negative voltage is on the attenuator 300 side and a positive voltage is on the first glass member 110 side, the generated electron flow e means a reverse voltage at the joint portion 510. Because of this reverse voltage, reduction of joint strength, detachment of the joint surface and the like may occur at the joint portion 510, which in turn may cause deterioration in sensor accuracy. It should be noted that the joint portions 610,620 correspond to the joint portion 600 in FIG. 19B.
Therefore, first, it would be desirable to provide a force sensor in which the positioning of the glass member is facilitated when the force sensor chip and the attenuator are joined through the glass member.
Second, it would be desirable to provide a force sensor in which joint strength is prevented from being reduced in a case where the force sensor chip and the attenuator are joined through the glass member by anodic bonding.